Chemical vapor deposition (CVD) systems are used to form a thin, uniform layer or film on a substrate such as a semiconductor silicon. During CVD processing, the substrate is exposed to one or more gaseous substances such as silane, phosphane, diborane, oxygen, ozone and the like, and chemical vapors such as TEOS (tetraethylorthosilicate), TMB (trimethylborate), TMPi (trimethylphosphite), TEB (trimethylborate), TEPo (triethylphospate) and the like. The gases are injected into a clean, isolated reaction chamber and allowed to mix and interact with the other gases and/or the surface of the substrate to produce the desired film. The CVD systems typically employ injectors which deliver the gaseous substances directly to the surface of the substrate. An exhaust system removes waste products, such as unreacted gases and powders formed during the reaction, from the reaction chamber. Over time, films are deposited on the exposed surfaces of the chamber creating sources of particulate contamination which may become embedded in the film or degrade film uniformity. In many applications including semiconductor processing, film characteristics such as purity and thickness uniformity must meet high quality standards. To preserve film quality and prevent unacceptable defect levels, the reaction chamber must be cleaned to remove the deposited films.
The injection ports are typically positioned less than one inch from the surface of the substrate. With this limited clearance between the injector and the substrate surface, the surfaces of the injector and chamber walls will become coated with the material produced during the reactions. To reduce the amount of build-up in this area, some CVD systems include shields which are positioned in front of the injectors and exhaust port. The shields include a perforated screen welded to a support body. Supply tubes deliver an inert gas such as nitrogen to the volume between the support body and the screen. The nitrogen exits the shield through the perforated screen to slow the rate at which materials accumulate on the shield during processing.
The desired reactions for chemical vapor deposition typically occur at elevated temperatures, for example 300.degree. C. to 600.degree. C., with the substrate and chamber being heated to the appropriate temperature for a selected process. The high temperatures in the reaction chamber create thermal stresses in the perforated screen which may cause the screen to buckle after a period of time. The thermal deformation of the perforated screen disrupts the uniform flow of nitrogen through the screen, leaving portions of the screen unprotected against the accumulation of deposition materials. The ability of the screen to deliver nitrogen to the reaction chamber is further reduced as the screen becomes coated with deposition materials, requiring removal and cleaning or replacement of the shield. Since the screen essentially defines an upper "wall" of the reaction chamber, the deformed screen also interferes with the uniformity and distribution of the process reactant chemistries within the reaction chamber. The delays created by removal of the shield for cleaning or the replacement of a damaged shield are time consuming and expensive. A shield in which thermal deformation of the screen is minimized or eliminated is desirable. A shield which provides a uniform supply of the inert gas to the reaction chamber is also desirable. A shield in which a damaged screen surface can be quickly and inexpensively replaced is similarly desirable.
For atmospheric pressure CVD (APCVD) processing, the substrates are transported during processing by a conveyor which carries the substrates through one or more reaction chambers. The reaction chamber is not an enclosed chamber, but is merely the area in front of the injector between a series of curtains which use an inert gas such as nitrogen to isolate the reaction chamber from the rest of the process path. The exhaust vents on either side of the injector are used to extract unused gases and reaction by-products from the reaction chamber. If the exhaust is extracted at a rate slower than the rate at which the gases are introduced to the reaction chamber, some of the reactants may escape from the reaction chamber. Thus, with prior art systems the flow rate of the exhaust is typically greater than the rate at which gases are injected into the chamber, with excess inert gas being drawn into the reaction chamber from the area between the reaction chambers to provide a buffer zone blocking the escape of reactant gases. However, the gas drawn into the chamber from the adjacent buffer zones is not uniformly metered across the width of the reaction chamber. Thus, a non-uniform gas-to-gas boundary is created along the width of the reaction chamber. A shield which effectively prevented the escape of reactant gases from the reaction chamber without interfering with the reaction chemistries is desirable. As gases are pulled into the exhaust vent from the area below the injector on one side of the vent and the buffer zone between the reaction chambers on the other side of the vent, a large volume of reactant gas recirculation is created between the opposing flow streams. A shield which efficiently exhausts reactant gases from the chamber and minimizes the amount of gas recirculation within the reaction chamber is desirable.